The present invention relates to positive electrode active material and lithium secondary batteries. More specifically, the present invention relates to lithium layered composite oxide advantageous in capacity as compared to a conventional spinel type lithium manganese composite oxide, and in high temperature cycle stability a conventional lithium manganese layered oxide, as positive electrode active material for rechargeable nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary batteries using, as positive electrode active material, such a lithium layered composite oxide.
Among various rechargeable secondary batteries, the lithium secondary battery with its high charge-discharge voltage and large charge-discharge capacity has shown much promise as source of electricity for electric vehicles to meet the recent intense demand for zero-emission vehicles in environmental problem.
In consideration of some aspects of LiCoO2 such as the stability in operating environment, cost and natural reserves of LiCoO2 used as positive electrode active material for a lithium secondary battery, investigation is currently under way on spinel structure lithium manganese composite oxide (LiMn2O4) as positive electrode active material of a secondary battery for an automotive vehicle. Japanese Published Patent Applications, Publication (Kokai) Nos. 11(1999)-171550 and 11(1999)-73962 show spinel structure lithium manganese composite oxides as positive electrode active material of a secondary battery.
However, LiMn2O4 as positive electrode active material is deficient in durability at high temperatures and liable to cause deterioration in performance of the negative electrode due to dissolution of the positive electrode material into the electrolyte. To meet these problems, technique is on trial, of substituting various elements such as transition metals and typical metallic elements, for part of Mn. However, the partial substitution of various element for Mn to improve the high temperature cycle durability as disclosed in Published Japanese Patent Application, Publication (Kokai) No. 11(1999)-71115 is liable to cause distortion in the crystal structure and hence deteriorate the cycle durability at room temperature. Moreover, an increase in the amount of substitution for further improvement of the stability of the crystal structure tends to lower the capacity of the active material.
A lithium manganese composite oxide in an amorphous state by the content of K as disclosed in Japanese Published Patent Applications, Publication (Kokai) Nos. 11(1999)-297323 is liable to cause a decrease in the active material capacity at high temperatures above room temperature.
As to the capacity, lithium cobalt oxides (LiCO2: the active material capacity=140 mAh/g) are higher in capacity than spinel type lithium manganese composite oxides (LiMn2O4: the active material capacity=100 mAh/g). However, lithium cobalt oxides are disadvantageous in the stability etc., as mentioned before. Therefore, a desired positive electrode active material is a high-capacity Mn containing lithium composite oxide which is higher in the Li content in the crystal structure than the spinel lithium manganese composite oxides (LiMn2O4) and which is superior in stability in operating environment to the lithium cobalt oxides.
In such a high-capacity type positive electrode active material for a lithium secondary battery, the lithium content in a chemical formula based on the crystal structure is determinative. Japanese Patent 2870741 seeks for a high-capacity Mn containing lithium composite oxide on the basis of crystal-chemical studies.
A recent report (A. Robert Armstrong and P. G. Bruce xe2x80x9cSynthesis of layered LiMnO2 as an electrode for rechargeable lithium batteriesxe2x80x9d, Nature, vol.381 (1996) p499) reveals LiMnO2 layered oxide has a positive electrode active material capacity of about 270 mAh/g, more than twice of that of a conventional spinel structure lithium manganese oxide.
With this layered oxide, a sufficient charge-discharge characteristic is obtainable at 55xc2x0 C., for example. However, the active material capacity decreases to about one third at room temperature. Moreover, the capacity is decreased gradually by repetition of charge and discharge at temperatures over room temperature, so that the cycle durability is insufficient.
It is therefore an object of the present invention to provide a lithium manganese layered composite oxide positive electrode active material which is higher in capacity than the conventional spinel structure lithium manganese composite oxide, and advantageous in high temperature cycle durability as compared to the conventional layered structure lithium manganese composite oxide, and to provide a high-performance lithium secondary battery using this high-capacity lithium manganese layered composite oxide.
According to the present invention, a positive electrode active material for a nonaqueous electrolyte secondary battery comprises: a lithium-containing manganese composite oxide having a layered crystal structure represented by the general formula LiMeO2 where Me is a second constituent comprising Mn. The lithium-containing manganese composite oxide comprising a lithium substitute metal A substituting for a part of Li, and being represented by a chemical formula Li1-xAxMeO2.
The lithium-containing manganese composite oxide may be represented by the formula Li1-xAxMnO2 or Li1-xAxMn1-yMyO2. The lithium substitute metal A may comprise at least one selected from the group consisting of alkali elements and Ag, and a lithium substitution quantity x of the lithium substitute metal A may be in the range of 0.03xe2x89xa6xxe2x89xa60.2. The lithium-containing manganese composite oxide may further comprise a manganese substitute metal M substituting for a part of Mn.